Method and device to prevent coating a dovetail of a turbine airfoil

ABSTRACT

A method and masking assembly for masking a dovetail portion of a turbine blade during coating of an airfoil portion of the blade. The masking assembly comprises at least two masking members, each having an exterior surface and an oppositely-disposed undulatory surface complementary to one of oppositely-disposed undulatory surfaces of the dovetail portion. By mating the masking members, the undulatory surfaces thereof define an interior cavity within the masking assembly that accommodates the dovetail portion, and the undulatory surfaces of the masking members contact the undulatory surfaces of the dovetail portion to entrap the dovetail portion within the interior cavity of the masking assembly.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a Division Patent Application of co-pending U.S. patentapplication Ser. No. 11/276,745, filed Mar. 13, 2006.

BACKGROUND OF THE INVENTION

The present invention generally relates to coating deposition processesand equipment. More particularly, this invention relates to a method andmasking assembly for selectively depositing a coating on a turbineairfoil while preventing deposition of the coating on a dovetail of theairfoil.

Components of gas turbine engines, such as the blades and vanes(nozzles) of the turbine section within a gas turbine engine, are oftenformed of an iron, nickel, or cobalt-base superalloy. A turbine bladehas an airfoil against which hot combustion gases are directed duringoperation of the gas turbine engine, and whose surface is thereforesubjected to severe attack by oxidation, corrosion and erosion. Theblade further includes a root section separated from the airfoil by aplatform. Turbine blades are commonly anchored to the perimeter of arotor or wheel by forming the rotor to have slots with dovetailcross-sections, and forming the root section of each blade to have acomplementary dovetail profile whose oppositely-disposed undulatorysurfaces, generally characterized by alternating lobes and recesses,interlock with the dovetail slot of the rotor.

Due to the severity of their operating environments, turbine bladesoften require environmentally protective coatings on the surfaces oftheir airfoils and platforms exposed to the hot gas path. Diffusioncoatings such as chromide, aluminide, and platinum aluminide coatingsare widely used as environmental coatings in gas turbine engineapplications because of their oxidation resistance. Such coatings, whichare typically applied to the internal and external surfaces of a blade,are produced by a thermal/chemical reaction process that takes place ina reduced and/or inert atmosphere at a specified temperature. Commonprocesses include pack cementation and noncontact vapor (gas phasedeposition) techniques, and typically take place at processingtemperatures of about a 1900° F. (about 1040° C.) or more. The dovetailof a turbine blade is typically machined prior to the diffusion coatingprocess, and is not coated during coating of the airfoil so that thedovetail will properly assemble with the dovetail slot in the rotorduring engine build.

Slurries, putties, and tapes have been widely used as masks to preventcoating deposition on the machined surfaces of blade dovetails. Oneapproach is to cover the dovetail surfaces with a mask formed from aslurry paste, such as a mixture of nickle powders and an organic binder.The slurry paste may be applied with pneumatic injection equipment andthen dried to form a solid mask. Alternatively, the blade dovetails canbe dipped into the masking slurry, with multiple dips typically beingrequired to form an effective mask with sufficient thickness. The maskeddovetails are then often wrapped in a metal foil to contain the maskantduring the coating process. With either approach, the solid mask must bemechanically removed after the coating process, such as by gritblasting, rotating wire brush, etc. To avoid the requirement of removinga solid maskant, the dovetail can simply be buried in a nickle powderwithout any binder, so that the powder forms a loose maskant that coversthe dovetail during the coating operation. Still another alternative isto cast the slurry into thin film tapes that can be individually appliedto the blade. While this approach is well suited for masking localizedareas, tapes are not typically used as a primary method for masking theundulatory machined surfaces of a dovetail.

Significant shortcomings associated with the above-noted approachesinclude the preparation, application, and removal of the maskingmaterials, which can be labor intensive and require the services of askilled individual. As such, alternative masking techniques have beenproposed. On such approach is taught in commonly-assigned U.S. Pat. Nos.6,224,673, 6,579,567, and 6,821,564 to Das et al. These patents teachthe use of a reusable fixture to enclose those portions of an article,such as a gas turbine blade, on which a coating is not desired. Thefixture has an internal cavity and at least one aperture whosecross-section is substantially the same as a cross-section of thearticle to be coated. In the case of a blade, the aperture is sized toenable the entire dovetail of the blade to be inserted through theaperture into the internal cavity of the fixture, so that the platformseals against the exterior of the fixture. The fixture may include aholder to stabilize the dovetail within the internal cavity.

While the teachings of Das et al. overcome the shortcomings associatedwith the use of masking tapes, slurries, and other types of coatings,further improvements would still be desirable.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a method and masking assembly for maskinga dovetail portion of a turbine blade during coating of an airfoilportion of the blade. The masking assembly comprises at least twomasking members, each having an exterior surface and anoppositely-disposed undulatory surface complementary to one ofoppositely-disposed undulatory surfaces of the dovetail portion. Bymating the masking members, the undulatory surfaces thereof define aninterior cavity within the masking assembly that accommodates thedovetail portion, and the undulatory surfaces of the masking memberscontact the undulatory surfaces of the dovetail portion to entrap thedovetail portion within the interior cavity of the masking assembly.

The method of this invention generally entails placing the dovetailportion in the masking assembly so that the airfoil portion of the bladeremains outside the masking assembly, the interior cavity of the maskingassembly accommodates the dovetail portion, and the undulatory surfacesof the masking members contact the undulatory surfaces of the dovetailportion and entrap the dovetail portion within the interior cavity. Acoating vapor is then supplied to deposit a coating on the airfoilportion of the blade while preventing deposition of coating on thedovetail portion with the masking assembly. Following coatingdeposition, the masking members are separated to release the blade fromthe masking assembly.

In view of the above, the present invention provides a simplified methodfor masking the dovetail portion of a turbine blade, without therequirement for masking the dovetail portion with a masking slurry ortapes as conventionally done in the past. As such, the present inventioneliminates the labor required to prepare and apply a masking slurry overthe entire dovetail portion, and avoids the additional labor required tomechanically remove a solidified mask formed by the masking slurry atthe conclusion of the coating operation. As a result, both the maskingassembly and method made possibly with the masking assembly areconsiderably less complicated than prior art masking methods, yetachieves the object of preventing coating of the dovetail portion of aturbine blade.

Other objects and advantages of this invention will be betterappreciated from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a masking member in accordance with apreferred embodiment of this invention.

FIG. 2 is a perspective view of a turbine blade installed in a maskingassembly formed by mating two masking members of the type shown in FIG.1, such that a dovetail portion of the blade is enclosed within themasking assembly and an airfoil portion of the blade is exposed outsidethe masking assembly.

FIG. 3 is a cross-sectional view through the masking assembly of FIG. 2.

FIG. 4 represents the blade of FIGS. 2 and 3 after removal from themasking assembly.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a method for preventing the deposition ofa coating on surfaces of the dovetail portion of a gas turbine engineblade, particularly a turbine blade. While the advantages of thisinvention will be illustrated and described with reference to a turbineblade on which an environmental coating is to be deposited to protectthe blade from its hostile operating environment, the teachings of thisinvention are generally applicable to other components having surfacesand on which a coating and still other surfaces on which a coating isnot desired.

FIGS. 1 through 3 depict a masking shell 12 and a turbine blade 14installed in a masking assembly 10 formed by mating the masking shell 12with a second and essentially identical masking shell 12, resulting inthe assembly 10 having a clam shell-like construction. Blades of thetype represented in the Figures are typically formed of an iron, nickel,or cobalt-base superalloy, though the use of other materials is withinthe scope of this invention. The blade 14 includes an airfoil 16 againstwhich hot combustion gases are directed during operation of the gasturbine engine, and whose surfaces are therefore subjected to severeattack by oxidation, corrosion and erosion. The blade 14 is configuredto be anchored to a turbine disk (not shown) with a dovetail 18 formedon a root section of the blade 14. A platform 20 is between the airfoil16 and dovetail 18. The dovetail 18 has opposing surfaces 22 that may betermed undulatory, wavy, etc., in other words, generally characterizedby alternating lobes and recesses. As is known in the art, the dovetailsurfaces 22 are complementary to surfaces of a slot formed in the diskinto which the dovetail 18 will be inserted to interlock the blade 14with the disk.

The airfoil 16 is intended to be protected from the hostile environmentof the turbine section by an environmentally-resistant coating, forexample, a diffusion coating such as a chromide, aluminide, or platinumaluminide coating. As is understood in the art, these types of coatingsare formed by such processes as pack cementation or noncontact vapor(gas phase deposition) techniques, in which a vapor of a desired coatingelement (e.g., chromium, aluminum, etc.) is generated and caused tocontact the surfaces of the blade 14 on which the coating is desired.The vapor reacts with the surface to deposit the desired coatingelement(s), which are then diffused into the surface. Such processes arewell known to those skilled in the art, and therefore will not bediscussed in further detail here.

As noted above, the present invention is intended to prevent depositionof the coating on surfaces of the dovetail 18, particularly itsundulatory surfaces 22 that are required to subsequently mate with thedovetail slot in a rotor. For this purpose, most of the dovetail 18 isshown in FIGS. 2 and 3 as being enclosed within an interior cavity 24formed by mating the masking shells 12 to yield the masking assembly 10.The cavity 24 is defined by opposing interior surfaces 26 of the maskingshells 12. As evident from FIG. 3, the interior surfaces 26 of themasking shells 12 are complementary to the undulatory surfaces 22 of thedovetail 18, so that the surfaces 26 of the shells 12 contact andinterlock with the dovetail surfaces 22 to secure and essentiallyimmobilize the dovetail 18 within the masking assembly 10. While thesurfaces 26 of the masking shells 12 are represented as having a closeand continuous surface-to-surface fit with the surfaces 22 of thedovetail 18, it is foreseeable that the benefits of the invention couldbe realized without such a continuous surface-to-surface fit, as long assufficient contact exists to interlock and secure the dovetail 18 to themasking assembly 10.

FIG. 3 shows the masking assembly 10 as including an optional passage 40through which coating vapors can enter the interior of the blade 10 toenable deposition of the coating on any internal cooling passages withinthe blade 10. If necessary, a maskant material (not shown) of a typeused in the prior art as described previously, such as a tape, putty, orslurry, can be applied to mask any exterior surfaces of the dovetail 18exposed by the passage 40 in the masking assembly 10.

The masking assembly 10 is also represented in FIGS. 2 and 3 as furtherhaving a retaining ring 28 to secure the masking shells 12 together. Thering 28 preferably forces the mating surfaces of the shells 12 togetherwith sufficient force to close the split line 30 between the shells 12and prevent entrance of the coating vapors into the cavity 24. For thispurpose, the retaining ring 28 preferably has a draft angle machinedinto its surface contacting the masking shells 12 to ease itsinstallation and removal from the masking assembly 10 while locking thering 28 in place. FIG. 2 further shows a sealing material 32 depositedalong the interfaces 34 between the masking shells 12 and the blade 14to further inhibit coating penetration. The sealing material 32 may be,for example, a maskant material of a type used in the prior art asdescribed previously, such as a tape, putty, or bead of slurry.

Suitable materials for the masking shells 12 and retaining ring 28include metallic and ceramic materials. In practice, the nickel-basesuperalloy commercially known as Inconel 600 has been shown to be adurable and reusable material for both the shells 12 and ring 28, thoughit is foreseeable that other materials could be used. The surfaces ofthe masking shells 12 and ring 28 that contact other components of theassembly 10 or the blade 14 are preferably machined to ensure anappropriate fit.

In view of the above, masking of the dovetail 18 with the maskingassembly 10 simply involves placing the dovetail 18 in the assembly 10by mating the masking shells 12 so that the undulatory surfaces 26 ofthe shells 12 contact and entrap the dovetail 18 within the interiorcavity 24 of the masking assembly 10. After installing the retainingring 28, the entire blade and masking assembly can then be placed in asuitable coating apparatus (not shown) and ran through a coating cycleas required by the particular coating material and coating process beingemployed. Once the coating cycle is complete, the retaining ring 28 isremoved and the masking shells 12 separated to release the blade 14.Reuse of the masking shells 12 and retaining ring 28 may generallyinvolve removing the sealing material 32 and any residual coatingmaterial from the exterior surfaces of the assembly 10. The blade 14 isrepresented in FIG. 4 as having been removed from the assembly 10, andas having a coating 38 on only the surfaces of the airfoil 16 andplatform 20 and limited surface portions of the dovetail 18 exposed tothe coating vapor, with the dovetail 18 being free of coating below acoating boundary 36 corresponding to the uppermost extent of the maskingassembly 10 on the blade 14. From the location of the coating boundary36, it is evident that the masking assembly 10 of this invention enablesthe entire platform 20, including its upper and lower surfaces, to becoated for oxidation and corrosion protection.

While the invention has been described in terms of a preferredembodiment, it is apparent that other forms could be adopted by oneskilled in the art. For example, the physical configuration of themasking assembly 10, shells 12, blade 14, and ring 28 could differ fromthat shown. Therefore, the scope of the invention is to be limited onlyby the following claims.

1. A masking assembly for masking a dovetail portion of a turbine bladeduring coating of an airfoil portion of the blade, the masking assemblycomprising at least two masking members, each masking member having anexterior surface and an oppositely-disposed undulatory surfacecomplementary to one of oppositely-disposed undulatory surfaces of thedovetail portion whereby placing the dovetail portion in the maskingassembly by mating the masking members causes the undulatory surfacesthereof to define an interior cavity within the masking assembly thataccommodates at least part of the dovetail portion and the undulatorysurfaces of the masking members contact the undulatory surfaces of thedovetail portion to entrap the dovetail portion within the interiorcavity of the masking assembly.
 2. The masking assembly according toclaim 1, wherein the masking members are substantially identical.
 3. Themasking assembly according to claim 1, further comprising a retainingring securing the masking members together.
 4. The masking assemblyaccording to claim 1, wherein the masking members are formed of amaterial chosen from the group consisting of metallic and ceramicmaterials.
 5. The masking assembly according to claim 1, wherein themasking members are formed of a nickel-base superalloy.
 6. The maskingassembly according to claim 1, wherein the dovetail portion is withinthe interior cavity of the masking assembly and secured therein by theundulatory surfaces of the masking members.
 7. The masking assemblyaccording to claim 6, further comprising a sealant at interfaces definedby and between the masking members and by and between the dovetailportion and the masking members to prevent a coating vapor from enteringthe interior cavity of the masking assembly.
 8. The masking assemblyaccording to claim 7, wherein the sealant is a masking tape, putty, orslurry.
 9. The masking assembly according to claim 6, wherein the bladefurther comprises a platform between the airfoil and dovetail portions,and airfoil and the platform are outside the masking assembly to enablecoating of the airfoil and the platform in their entireties.
 10. Themasking assembly according to claim 6, wherein the blade furthercomprises an internal cooling passage and the masking assembly has apassage therein through which a coating vapor can enter the internalcooling passage.